Method of producing phase mask for fabricating optical fiber and optical fiber with bragg&#39;s diffraction grating produced by using the phase mask

ABSTRACT

The invention relates to a method of fabricating an optical fiber-processing phase mask in which a stitching error ascribable to a deterioration in the wavelength selectivity of the optical fiber diffraction grating to be fabricated is reduced. At an exposure step, a writing stage  5  with a phase mask blank  10  placed thereon is continuously fed in one direction while portions of the phase mask blank corresponding to grooves  26  or strips  27  in a direction perpendicular to the direction of feeding are sequentially scanned with writing beams  14 , whereby the entire area of the phase mask blank  10  to be written is continuously written.

TECHNICAL ART

The present invention relates to a method of fabricating a phase maskfor processing optical fibers, and an optical fiber with a Braggdiffraction grating, which is manufactured using the opticalfiber-processing phase mask. More particularly, the present inventionrelates to a method of fabricating a phase mask for making a diffractiongrating in an optical fiber used for optical communications, etc. usingultraviolet laser light, and an optical fiber with a Bragg diffractiongrating, which is manufactured using the mask.

BACKGROUND ART

Optical fibers have brought about breakthroughs in the globalization ofcommunications to make high-quality and large-capacity inter-oceanictelecommunications feasible. So far, it has been known that a Braggdiffraction grating is provided in an optical fiber by creating aperiodic refractive index profile in an optical fiber core along theoptical fiber, and the magnitude of reflectivity and the width of thewavelength characteristics of the diffraction grating are determined bythe period and length and the magnitude of refractive index modulationof the diffraction grating, whereby the diffraction grating can be usedas a wavelength division multiplexer for optical communications, anarrow-band yet high-reflection mirror used for lasers or sensors, awavelength selection filter for removing extra laser wavelengths infiber amplifiers, etc.

However, the wavelength at which the attenuation of a silica opticalfiber is minimized and which is suitable for long-distancecommunications is 1.55 μm. It is thus required that the grating spacingbe about 500 nm in order to allow the optical fiber diffraction gratingto be used at this wavelength. At the beginning, it was considereddifficult to make such a minute structure in an optical fiber core; thatis, a Bragg diffraction grating was provided in the optical fiber coreby a sophisticated process comprising a number of steps, e.g., sidepolishing, photoresist coating, holographic exposure, and reactive ionbeam etching. For this reason, much fabrication time was needed,resulting in low yields.

In recent years, however, a method of fabricating a diffraction gratingby irradiating an optical fiber with ultraviolet radiation to cause arefractive index change directly in an optical fiber core has beendeveloped. This ultraviolet irradiation method has been steadily put topractical use with the advance of peripheral technologies, because of noneed of any sophisticated processes.

Since the grating spacing is as fine as about 500 nm as mentioned above,this method using ultraviolet light is now carried out by aninterference process using the interference of two light beams, awriting-per-point process wherein single pulses from an excimer laserare focused to make diffraction grating surfaces one by one, anirradiation process using a phase mask having a grating, etc.

Regarding the interference process using the interference of two lightbeams, a problem arises in conjunction with the quality of the beams inthe lateral direction, i.e., spatial coherence. A problem with thewriting-per-point process is on the other hand that strict step controlof the submicron order is needed to focus light on a small point forwriting light on many surfaces. Another problem arises in conjunctionwith processability.

To solve these problems, attention has focused on the irradiationprocess using a phase mask. According to this process, a phase mask 21comprising a quartz substrate provided on one surface with grooves ofgiven depth at a given pitch is irradiated with ultraviolet laser light(of 190 to 3.00 nm wavelength) 23 to give a refractive index. change toa core 22A of an optical fiber 22, thereby producing a grating(diffraction grating)., as shown in FIG. 5(a). For a betterunderstanding of an interference pattern 24 Qn the core 22A, the pattern24 is exaggerated in FIG. 5(a). FIG. 5(b) is a sectional view of thephase mask 21, and FIG. 5(c) is a top view corresponding to FIG. 5(b).The phase mask 21 has a binary phase type of diffraction gratingstructure where a substrate is provided on one surface with grooves 26having a depth D at a repetition pitch P, with a strip 27 substantiallyequal in width to each groove being provided between adjacent grooves26.

The depth of each groove 26 on the phase mask 21 (the difference inheight between strip 27 and groove 26) D is chosen such that the phaseof the ultraviolet laser light (beam) that is exposure light ismodulated by π radian. Thus, zero-order light (beam) 25A is reduced to5% or less by the phase mask 21, and chief light leaving the mask 21 isdivided into +first-order diffracted light 25B containing at least 35%of diffracted light and − first-order diffracted light 25C, so that theoptical fiber 22 is irradiated with the +first-order diffracted light25B and − first-order diffracted light 25C to produce an interferencefringe at a given pitch, thereby providing a refractive index change atthis pitch in the optical fiber 22.

The grating produced in the optical fiber using such a phase mask 21 asmentioned above has a constant pitch, and so the phase mask 21 used forgrating production is provided with grooves 26 at a constant pitch.

Such a phase mask is produced by writing an electron beam on positions,corresponding to grooves 26, on the quartz substrate coated with anelectron-beam resist, using an electron-beam writing system and etchingout the written portions.

To achieve a narrow-band optical fiber diffraction grating, however,such a phase mask 21 as mentioned above is required to have a size ofthe order of 100 mm in the repetition direction of grooves 26 (in thesectional direction in FIG. 5). In addition, it is not easy tocontinuously expose a phase mask blank to writing beams in oneoperation. Thus, such an optical fiber diffraction grating is fabricatedby writing the entire region of a phase mask blank with writing beams bya step-and-repeat process wherein the entire region of the phase maskblank is divided into small segments (at an interval of about 7 mm). Onesegment is first written with writing beams while a writing stage isfixed, and then the writing stage is moved by one segment to write thenext segment with writing beams. This operation is repeated while thesegments are connected to one another, so that the entire region of thephase mask blank can be sequentially written with the writing beams.

However, a problem with this step-and-repeat process is that there is aphase shift (a stitching error) in the repetition period of grooves 26in the connection of adjacent segments to each other. In an opticalfiber diffraction grating fabricated using a phase mask having such astitching error, a number of unnecessary peaks other than essential sidelobes occur on both sides of the center Bragg peak, as can be seen fromthe wavelength vs. reflectivity relation shown in FIG. 8.

DISCLOSURE OF THE INVENTION

In view of such problems with the prior art as mentioned above, oneobject of the present invention is to provide a method of fabricating anoptical fiber-processing phase mask which can reduce or substantiallyeliminate stitching errors ascribable to a deterioration in thewavelength selectivity of the optical fiber diffraction grating to befabricated. The present invention also includes an optical fiber with aBragg diffraction grating, which is manufactured using such a phase maskfor processing optical fibers.

According to one aspect of the present invention, this object isaccomplished by the provision of a method of fabricating an opticalfiber-processing phase mask comprising a transparent substrate providedon one surface with a grating form of repetitive groove-and-strippattern, in which an optical fiber is irradiated with light diffractedby said repetitive pattern to produce an interference fringe byinterference of diffracted light of different orders, thereby providinga diffraction grating in the optical fiber, wherein:

at an exposure step, a writing stage with a phase mask blank placedthereon is continuously fed in one direction while a portion of saidphase mask blank corresponding to a groove or a strip perpendicular tosaid feeding direction is scanned with a writing beam, therebycontinuously writing said beam on an entire region of said phase maskblank to be written.

According to another aspect of the present invention, there is provideda method of fabricating an optical fiber-processing phase maskcomprising a transparent substrate provided on one surface with agrating form of repetitive groove-and-strip pattern, in which an opticalfiber is irradiated with light diffracted by said repetitive pattern toproduce an interference fringe by interference of diffracted light ofdifferent orders, thereby providing a diffraction grating in the opticalfiber, wherein:

when, at an exposure step, a writing beam is written on an entire regionof a phase mask blank to be written while segments smaller than saidentire region to be written are sequentially scanned with said writingbeam while said segments are connected to one another in a directionperpendicular to a groove or a strip, adjacent segments are allowed tooverlap one another at a part of end areas thereof.

In the present invention, beam writing may be carried out using eitheran electron-beam writing system or a laser-light writing system.

In the present invention, the pitch of the grating form of repetitivegroove-and-strip pattern is usually between 0.85 μm and 1.25 μm.

It is here noted that the difference in height between grooves andstrips in the grating form of repetitive groove-and-strip pattern ispreferably set such that the phase of optical fiber-processingultraviolet radiation is shifted by approximately π upon transmission.

The present invention also encompasses an optical fiber provided with aBragg diffraction grating, which is produced using the opticalfiber-processing phase mask fabricated by either one of the above twofabrication methods.

According to the present invention, the writing stage with the phasemask blank placed thereon is continuously fed in one direction at theexposure step while a portion of the phase mask blank corresponding to agroove or a strip perpendicular to the feeding direction is scanned witha writing beam, thereby continuously writing the entire region of thephase mask blank to be written. Alternatively, when, at the exposurestep, a writing beam is written on the entire region of a phase maskblank to be written while segments smaller than said entire region to bewritten are sequentially scanned with said writing beam while saidsegments are connected to one another in a direction perpendicular to agroove or a strip, adjacent segments are allowed to overlap one anotherat a part of end areas thereof. Thus, there is no stitching error due toconnections between the segments to be written, unlike the prior art. Inthe optical fiber provided with a Bragg diffraction grating which isproduced using such a phase mask, no unnecessary peaks occur on bothsides of the center Bragg peak.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic illustrative of how the fabrication method of theinvention is carried out using an electron-beam writing system.

FIG. 2(a) is illustrative of an electron-beam writing process used withthe method of fabricating a phase mask, and FIG. 2(b) is a sectionalview of the phase mask.

FIG. 3 is a graph showing one wavelength vs. reflectivity relation ofthe optical fiber provided with a Bragg diffraction grating, which isproduced using the phase mask of the invention. FIGS. 4(a), 4(b), 4(c),4(d), 4(e), 4(f), 4(g) and 4(h) are sectional views of steps in oneembodiment of the chase mask fabrication method according to theinvention.

FIGS. 5(a), 5(b) and 5(c) are views illustrative of optical fiberprocessing and a phase mask used therewith.

FIG. 6 is a schematic illustrative of another writing process accordingto the invention.

FIG. 7 is an exaggerated view of a part of the overlapping region inFIG. 6.

FIG. 8 is a graph illustrative of one wavelength vs. reflectivityrelation of an optical fiber with a Bragg diffraction grating, which isfabricated by a conventional step-and-repeat process.

BEST MODE OF CARRYING OUT THE INVENTION

The method of fabricating an optical fiber-processing phase maskaccording to the present invention will now be explained with referenceto some preferred embodiments.

FIG. 2(b) is a sectional view of a part of a phase mask 21 in itslongitudinal direction. This phase mask 21 comprising a repetitivealternate groove-and-strip pattern is located as shown in FIG. 5(a) forthe purpose of fabricating a Bragg diffraction grating in an opticalfiber. The groove is shown at 26, and the strip is shown at 27. As shownin the FIG. 2(a) top view, one groove 26 is formed on a quartz substratewith an electron-beam resist coated thereon (see FIG. 4) by raster scanof the substrate with electron-beam scanning lines 28 in the widthwisedirection. As shown by broken lines, one strip 27 is formed on thesubstrate by blanking the electron-beam scan. According to the presentinvention, the whole of the continuous mask 21 is exposed to electronbeams as follows. Raster scan is carried out in a direction shown by adouble-arrow in FIG. 2(a) (in a direction vertical to groove 26 andstrip 27). As mentioned above, the position of the substrate on whichone groove 26 is to be written is actually scanned with a given numberof scanning lines (five lines in FIG. 2(a)), and at the position of thesubstrate on which the next strip 27 is to be written as many asscanning lines are blanked. This is repeated sequentially over and overagain, so that the phase mask 21 having a given length can be exposed toelectron beams in one operation.

In the present invention, an electron-beam writing system is used, whichis built up of an electron gun 1, an electronic lens 2 for convergingelectron beams 2 radiated from the electron gun 1, an electronicdeflector 4 for deflecting the converged electron beams 14, and awriting stage 5 movable in a direction (Y-direction) perpendicular toone direction (X-direction) of scanning of the converged electron beams14 deflected by the electronic deflector 4, as schematically shown inFIG. 1. On the writing stage 5 there is placed a phase mask blank 10comprising a quartz substrate coated thereon with an electron-beamresist. While the writing stage 5 is carried at a constant speed in thedirection (Y-direction) perpendicular to the direction of scanning, thephase mask blank 10 is scanned with the converged electron beams 14 at agiven repetition interval in the direction of scanning (X-direction) towrite grooves 26 thereon with the electron beams.

In the present invention as mentioned above, the phase mask blank isplaced on the writing stage at the exposure step of the lithographicprocess of fabricating an optical fiber-processing phase mask. Then,while the writing stage is continuously fed in one direction, portionsof the phase mask blank corresponding to grooves or strips perpendicularto the feeding direction of the writing stage are sequentially scannedwith writing beams, so that the entire area of the phase mask blank tobe written can be continuously written. Unlike the prior art, there isthus no stitching error due to connections between the written segments.Accordingly, such unnecessary peaks (noises) as shown in FIG. 8 are notfound in an optical fiber diffraction grating fabricated using thisphase mask.

The first aspect of the invention is explained with reference to onespecific example. A silica optical fiber with photosensitivity enhancedby the filling of high-pressure hydrogen was used as the photosensitiveoptical fiber 22, and continuously exposed to light by the aforesaidprocess. On the other hand, a phase mask 21 of 100 mm in length wasfabricated through the following steps. Then, the modulation ofrefractive index was imparted directly to a core 22A of the opticalfiber 22, as shown in FIG. 5(a). In this case, an argon SHG laser (of244 nm wavelength) was used as the ultraviolet laser light source. Thewavelength vs. reflectivity relation of the thus fabricated opticalfiber provided with a Bragg diffraction grating is shown in FIG. 3. Froma comparison with FIG. 8, it is seen that unnecessary peaks are notsubstantially found on both sides of the center Bragg peak.

FIG. 4(a) to FIG. 4(h) are sectional views of the process of fabricatingthe aforesaid phase mask 21. In these figures, reference numeral 10represents a phase mask blank, 11 a quartz substrate, 12 a chromium thinfilm, 12A a chromium thin-film pattern, 12B an opening in the chromiumthin film, 13 an electron-beam resist, 13A a resist pattern, 13B aresist opening, 14 converged scanning electron beams in the FIG. 1system, 21 a phase mask, 26 a groove, and 27 a strip.

As shown in FIG. 4(a), the blank 10 was first prepared by forming thechromium thin film 12 of 150 Å in thickness on the quartz substrate 11.The chromium thin film 12 is useful for preventing a charging-up of theelectron-beam resist 13 at the electron-beam (14) irradiation step, andserves as a mask in the formation of the groove 26 on the quartzsubstrate. The thickness of this chromium thin film is important in viewof resolution in chromium thin-film etching, and so should preferably becontrolled to 100 to 200 Å.

Then, electron-beam resist RE5100P (made by Hitachi Kasei Co., Ltd.) asthe electron-beam resist 13 was coated on the chromium thin film 12 to athickness of 400 nm, and dried, as shown in FIG. 4(b).

After this, the electron-beam resist 13 was exposed to light at anexposure of 1.2 μC/cm², as shown in FIG. 4(c), using an electron-beamwriting system MEBESIII (made by ETEC). That is, as explained withreference to FIGS. 1 and 2, one groove 26 was written with five rasterscanning lines. While the writing stage 5 was fed in one direction, theportions of the phase mask blank 10 corresponding to grooves 26perpendicular to the direction of feeding are sequentially scanned withthe electron beams 14 for exposure to the writing light of the entirearea of the phase mask blank 10 to be written.

After the exposure, post-exposure baking was carried out at 90° C. for 5minutes, and the electron-beam resist 13 was developed with TMAH(tetramethylammonium hydroxide) at a concentration of 2.38%, therebyforming such desired resist pattern 13A as shown in FIG. 4(d). It ishere noted that the post-exposure baking is to selectively enhance thesensitivity of the portion irradiated with the electron beams 14.

Then dry etching was performed with CH₂Cl₂ gas while the resist pattern13A was used as a mask, thereby forming such chromium thin-film pattern12A as shown in FIG. 4(e).

Subsequently, the quartz substrate 11 was etched with CF₄ gas to a depthof just 240 nm, while the chromium thin-film pattern 12A was used as amask, as shown FIG. 4(f). Depth control was conducted by etching timecontrol. Etching can occur while the etching depth is controlled in therange of 200 to 400 nm.

Following this, the resist pattern 13A was stripped off with sulfuricacid at 70° C., as shown in FIG. 4(g). Finally, the chromium thin-filmpattern 12A was etched out with an ammonium ceric nitrate solution, asshown in FIG. 4(h), and scrubbing was carried out to obtain a completeline-and-space phase mask 21 having a depth of 240 nm and a pitch of1.070 μm, wherein the lines and spaces corresponded to strips 27 andgrooves 26, respectively.

The above example is given to explain the lithographic process offabricating the phase mask 21 wherein while the writing stage with aphase mask blank placed thereon is fed in one direction, the entire areaof the phase mask blank to be written is sequentially scanned andwritten with writing beams. The present invention is also applicable toa step-and-repeat writing process wherein the entire area of a phasemask blank to be written is divided into small segments. First, onesegment is written while a writing stage is fixed in place:. Then, thewriting stage is moved by one segment so that the next segment iswritten. This operation is repeated to sequentially write the segmentswith writing beams while adjacent: segments are connected to each other.If, in this process, the movement of the writing stage is limited tosmaller than one segment to stitch one segment to another duringexposure, it is then possible to reduce stitching errors. This processis explained below.

FIG. 6 is a schematic view of segments S₁ to S₅ to be written. Thesegments S₁ to S₅ are connected to one another while they aresequentially written by the step-and-repeat process. In-this case, theadjacent segments are written while they overlap at an end region A.FIG. 7 is an exaggerated view of the overlapping region A. In FIG. 7,reference numeral 26 ₁ represents a portion of one of the adjacentsegment exposed to writing light to form a groove therein (solid lines),and 26 ₂ stands for a portion of another segment exposed to writinglight to form a groove therein (broken lines). Here assume that Δrepresents a displacement between the adjacent segment. Then, one groovedefines an overlap of the exposed portions 26 ₁ and 26 ₂, and the centerthereof displaces by Δ/2 from the center of a groove in the case wherethe adjacent segments do not overlap. That is, this displacementcorresponds to a half of the position displacement between the adjacentsegments. For this reason, the stitching error reduces to half. When theadjacent segments. overlap three times, the stitching error reduces to⅓. It is desired that the process of connecting segments to one anotherusing the overlapping region A be applied to the case where segments areconnected to one another not only in a direction perpendicular to thegroove but also in a direction along the groove. It is here to be notedthat when segments are written with electron beams while they overlap,the exposure intensity of the electron beams used for writing theoverlapping region should preferably be reduced depending on the numberof overlapping, so that the exposure quantity of the overlapping regioncan be much the same as that of a region which is exposed once toelectron beams.

While the method of fabricating an optical fiber-processing phase maskaccording to the invention and the optical fiber with a Braggdiffraction grating, which is manufactured using this opticalfiber-processing phase mask, are explained with reference to preferredembodiments, it is understood that the invention is not limited to theseembodiments, and so may be modified in various forms. While the presentinvention is explained with reference to the use of raster scan typeelectron-beam writing system, it is understood that the invention mayalso be carried out using a vector scan type writing system or the like.In the invention, a laser-light writing system may be used in place ofthe electron-beam writing system.

Industrial Applicability

In the method of fabricating an optical fiber-processing phase maskaccording to the invention as explained above, the writing stage withthe phase mask blank placed thereon is continuously fed in one directionat the exposure step while portions of the phase mask blankcorresponding to grooves or strips perpendicular to the direction offeeding are sequentially scanned with writing beams, therebycontinuously writing the entire region of the phase mask blank to bewritten with the writing beams. Alternatively, when, at the exposurestep, the entire region of the phase mask blank to be written arewritten with writing beams while segments smaller than the entire regionof the phase mask blank are sequentially scanned with the writing beamsand the segments are connected to one another in a directionperpendicular to grooves or strips, adjacent segments are allowed tooverlap one another at a part of end areas thereof. Thus, there is nostitching error due to connections between the segments to be written,unlike the prior art. In the optical fiber provided with a Braggdiffraction grating which is produced using such a phase mask, nounnecessary peaks occur on both sides of the center Bragg peak.

What we claim is:
 1. A method of fabricating an optical fiber-processingphase mask comprising a transparent substrate provided on one surfacewith a grating form of repetitive groove-and-strip pattern, in which anoptical fiber is irradiated with light diffracted by said repetitivepattern to produce an interference fringe by interference of diffractedlight of different orders, thereby providing a diffraction grating inthe optical fiber, wherein: at an exposure step, a writing stage with aphase mask blank placed thereon is continuously fed at a constant speedin one direction while portions of said phase mask blank correspondingto ones of plural grooves or strips perpendicular to said feedingdirection are scanned with plural writing beams, thereby continuouslywriting said beams on an entire region of said phase mask blank to bewritten.
 2. The method according to claim 1, wherein writing is carriedout using an electron-beam writing system.
 3. The method according toany one of claim 1 or 2, wherein said grating form of repetitivegroove-and-strip pattern has a pitch between 0.85 μm and 1.25 μm.
 4. Themethod according to any one of claim 1 or 2, wherein a difference inheight between grooves and strips in said grating form of repetitivegroove-and-strip pattern is set such that a phase of opticalfiber-processing ultraviolet radiation is shifted by approximately πupon transmission.
 5. An optical fiber provided with a Bragg diffractiongrating, which is produced using the optical fiber-processing phase maskfabricated by the method according to any one of claim 1 or
 2. 6. Themethod according to claim 3, wherein a difference in height betweengrooves and strips in said grating form of repetitive groove-and-strippattern is set such that a phase of optical fiber-processing ultravioletradiation is shifted by approximately π upon transmission.
 7. An opticalfiber provided with a Bragg diffraction gratings, which is producedusing the optical fiber-processing phase mask fabricated by the methodaccording to claim
 3. 8. An optical fiber provided with a Braggdiffraction grating, which is produced using the opticalfiber-processing phase mask fabricated by the method according to claim4.
 9. An optical fiber provided with a Bragg diffraction grating, whichis produced using the optical fiber-processing phase mask fabricated bythe method according to claim
 6. 10. The method of fabricating anoptical fiber-processing phase mask according to claim 1, wherein thenumber of said plural writing beams is at least five.